Flexible disks having a coating of magnetic material are widely used for the storage of digital information. These disks, known as "floppy" disks, are normally stored in conventional types of files in a lightweight cardboard or heavy paper envelope, which permits the floppy disk to be rotated and accessed while it remains in its container. Floppy disk drives, which are made by many manufacturers, rotate the disks while a magnetic head is in contact with the surface of the disk, for "reading" or "writing" magnetically coded information on its surface. The floppy disk drive also includes arrangements for shifting the magnetic head radially to successive "tracks" which define adjacent circular paths on the surface of the magnetic disk. The radial position of the magnetic head may be determined by a lead screw, for example, which may be rotated by the stepper motor. For a number of years, the spacing between adjacent tracks was in the order of 48 tracks per inch; but in the last few years, the track density has been increased, in many manufacturers' floppy disk drives, to in the order of 96 or 100 tracks per inch, and these higher density track systems are known as "double density" systems. These double density systems were pioneered by the assignee of the present invention, and one such system is disclosed in U.S. patent application Ser. No. 863,309, filed Dec. 22, 1977, and in the continuation patent application filed May 25, 1979, under Ser. No. 042,721, with both cases being entitled "High Precision Floppy Disk Memory System".
Now, in shifting from one track to another track in floppy disk systems of the type noted hereinabove, the normal track-to-track access speed is approximately 30 milliseconds. While somewhat higher track-to-track access speeds have been accomplished, they have usually been used with the lower track density systems, and have required expensive high accuracy stepper motors and have used lower accuracy and repeatability cam or band positioners. Further, these high speed track shifting arrangements have proved to be marginal in the double density systems. For the high density systems, increased speed has been accomplished using very expensive systems including "add-on" sensors in conjunction with closed loop positioning systems and active damping systems, all of which makes for a very expensive system.
Accordingly, a principal object of the present invention is to increase the track-to-track access of high density floppy disk memory systems without significantly increasing their cost.
A subordinate object of the present invention is to accomplish the foregoing object using the simple available geometry of a four-phase permanent magnetic stepper motor.